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MAX17058 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX17058 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 16 page MAX17058/MAX17059 1-Cell/2-Cell Li+ ModelGauge ICs 7 Maxim Integrated Detailed Description ModelGauge Theory of Operation The MAX17058/MAX17059 ICs simulate the internal, non- linear dynamics of a Li+ battery to determine its state of charge (SOC). The sophisticated battery model consid- ers impedance and the slow rate of chemical reactions in the battery (Figure 2). The ModelGauge algorithm performs best with a custom model, obtained by characterizing the battery at multiple discharge currents and temperatures to precisely model it. Contact Maxim if you need a custom model. At power- on reset (POR), the ICs have a preloaded ROM model that performs well for some batteries. Fuel-Gauge Performance In coulomb counter-based fuel gauges, SOC drifts because offset error in the current-sense ADC measure- ment accumulates over time. Instantaneous error can be very small, but never precisely zero. Error accumulates over time in such systems (typically 0.5%–2% per day) and requires periodic corrections. Some algorithms cor- rect drift using occasional events, and until such an event occurs the algorithm’s error is boundless: • Reaching predefined SOC levels near full or empty • Measuring the relaxed battery voltage after a long period of inactivity • Completing a full charge/discharge cycle The ModelGauge algorithm requires no correction events because it uses only voltage, which is stable over time. As the SOC accuracy without full/empty/relax shows the algorithm remains accurate despite the absence of any of the above events; it neither drifts nor accumulates error over time. To correctly measure performance of a fuel gauge as experienced by end-users, exercise the battery dynami- cally; accuracy cannot be fully determined from only simple cycles. Battery Voltage and State-of-Charge The open-circuit voltage (OCV) of a Li+ battery uniquely determines its SOC; one SOC can have only one value of OCV. In contrast, a given VCELL can occur at many dif- ferent values of OCV because VCELL is a function of time, OCV, load, temperature, age, and impedance, etc.; one value of OCV can have many values of VCELL. Therefore, one SOC can have many values of VCELL, so VCELL can- not uniquely determine SOC. Figure 3 shows that VCELL = 3.81V occurs at 2%, 50%, and 72% SOC. Even the use of sophisticated tables to consider both voltage and load results in significant error due to the load transients typically experienced in a system. During charging or discharging, and for approximately 30min after, VCELL and OCV differ substantially, and VCELL has been affected by the preceding hours of battery activity. ModelGauge uses voltage comprehensively by using voltage measured over a long period of time. Figure 2. Block Diagram Figure 3. Instantaneous Voltage Does Not Translate Directly to SOC STATE MACHINE (SOC) I2C INTERFACE IC GROUND TIME BASE (32kHz) ADC (VCELL) VOLTAGE REFERENCE BIAS GND CELL VDD SCL SDA ALRT QSTRT MAX17058 MAX17059 TIME (Hr) 100% 80% 60% 40% 20% 0% 012345678 3.4V 3.6V 3.8V 4.0V 4.2V 3.2V 3.81V = 2% 3.81V = 72% 3.81V = 50% 3.81V VCELL SOC |
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Similar Description - MAX17058_13 |
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